We've got a fair set of house rules that have evolved over the past four years. The lack of a reasonable "seriously injured" range in the whole hit point system bugged my group. At low level, the whole 0=disabled/-1 to -9=dying works so-so, but past 4th or 5th level it's a complete joke. Most attacks on higher level PCs will do well over 10 points of damage, so it's always a case of "I'm fine... I'm fine... I'm fine... I'm dead." We solved this by significantly increasing the disabled (from 0 on down), dying (from -Con on down) ranges and ruling that death occurs at -(Con+10) or -max hp, whichever is better for the character. To counterbalance for the leniency on death by hit points, we added a grievous injury system (i.e. long-term injuries, limb loss, brain damage and such - a simple d20 table - every time an injury brings your hit point total to dying or less, you roll once... having a monster savage a downed PC can do some horrible things). A recent fine tuning of this system (as the players increase in level) has led to us having dying characters lose 1hp/2HD rather than just 1 per round. Yes, we had to rewrite the non-lethal damage system and the Diehard feat to match.
Net result - Players feel fairly confident until they start getting in the low hit points, then they start looking for ways to withdraw. Once they dip into the negatives, they are disabled and their fighting capacity is greatly reduced. The regenerate spell has replaced raise dead as the big spell to get cast on battlefield casualties. Raise dead has been made much much harder and more painful to obtain.
So far, many PCs have been regenerated from horrific battle damage (i.e. crippled for life kind of injuries) but not a single dead PC has ever returned from the grave. Four dead PCs after 1 year, two from hit point damage (a psion who got grappled and breathed on by a red dragon and a fighter who didn't know when to quit against a clay golem) and two from massive damage (a psion and a mystic theurge who horribly fumbled a series of rolls while climbing a cliff-face... both failed massive damage saves and the theurge was also toast due to hit point damage if he had passed).
Other minor house rules have included forcing anyone casting an area effect spell to pick a square rather than an intersection and having the spell centre upon one randomly determined (d4 roll) corner of that square. Our reasoning is that fireballs aren't pinpoint precision spells and if you drop one too close to your buddies you just might misjudge your aim by a few feet and fry a friend. This rule has seriously reduced the amount of over-analyzing players would do over where the absolute best place to drop an area effect spell was. Now they look for a clear shot and don't sweat the details too much. Amazing what a little randomness does to speed things up.
Some spells have been houseruled, either for clarity or because we agreed the spell was poorly designed. The most common cases are no-SR, instantanous, energy based "conjuration" attack spells that appear in MiniHB, Complete series and other WotC expansion books. If it looks like an evocation, quacks like and evocation and walks like an evocation, it's an evocation and it gets SR just like other evocations. Just relabelling cone of cold as a conjuration and making it fire-based shouldn't make it suddenly ignore SR and be one spell level lower. A small number of PHB spells are houseruled, but mostly only for clarifications to oddball situations that came up in our games (e.g. what happens when a large creature is only partly in solid fog? we ruled it was partly impaired).
We also use poker chips for bonus XP. Everyone gets 3 chips at the start of the evening and the DM gets as many as he wants (I usually keep 7-8 in front of me). Each chip is worth 1% of a level and players may award them to each other as they see fit. So far, I've seen few abuses. Some players are more prone to hand out chips than others but it hasn't caused any significant issues.
As for multiclassing, sure, I'll allow paladins and monks to multiclass when they can explain it in such a way that is compatible with their overall goals as a paladin/monk. E.g. our paladin has multiclassed as a marshall to become a better leader of the armies of the faithful in an upcoming clash between good and evil; our monk is an active member of his church's clergy and multiclassed as a cleric of that faith. Both stayed true to their role as paladin and monk. The other class simply compliments their abilities and helps them further themselves in their roleplaying as paladins and monks. When the other class clashes or otherwise doesn't fit, I won't allow the multiclass without leaving the paladin/monk class.
